The present invention relates to devices for warming physiological fluids and more particularly this invention provides a device for warming physiological fluids at a location which is closely adjacent to the point at which the fluid enters a patient.
Many physiological fluids, such as whole blood, packed red blood cells, and fresh frozen plasma are stored at cold temperatures as low as 4.degree. C. until they are needed for a transfusion. While these fluids may be administered cold in small volumes or at low flow rates, for example, quantities of less than four 500 cc units/hr, as cold fluids are administered at higher flow rates there is a danger that the fluid will cause the patient's body temperature to fall, which may lead to cardiac dysfunctions or aggravate existing conditions such as shock or hypothermia. Thus, it is desirable to warm the fluid to a normal physiological temperature of approximately 37.degree. C. prior to administration of the fluid to the patient.
In order to administer physiological fluids at acceptable infusion temperatures several devices have been proposed. U.S. Pat. Nos. 4,705,505 and 4,759,749 to Fried disclose an administration set including a heat exchanger to heat the fluid, in which the fluid is directed through multiple aluminum rods and water is directed around the outside of the rods to transfer heat to the fluid. As the fluid leaves the heat exchanger, its temperature is measured and it subsequently flows through 8 inches of tubing, a drip chamber, and an additional 18 inches of tubing prior to infusion into the patient. Thus, the fluid flows through over two feet of tubing in addition to a drip chamber after the temperature is monitored at the heat exchanger, during which time the fluid temperature may decrease an indeterminate amount as a result of heat transfer through the tubing and drip chamber to the surroundings.
In U.S. Pat. No. 4,759,749, issued to Verkaart, a heater for physiological fluids is disclosed in which a heat exchanger for the device is designed to be supported on a pole. The heat exchanger includes a central tube positioned within an outer tube, wherein a heating fluid is circulated through the central tube and the physiological fluid is heated as it flows along the outside of the central tube. After leaving the heat exchanger, the fluid flows through two additional lengths of tubing and a filter during which time its temperature may fall as it loses heat to the surroundings.
U.S. Pat. No. 4,531,941 to Zasuwa discloses a device for heating blood in which the blood is passed through multiple plastic capillary tubes while heated water is circulated around the tubes. The output temperature of the blood leaving this device does not match the input temperature of the warming fluid, in addition to varying with the input temperature of the blood, and thus makes it difficult to accurately control the blood temperature by means of a preset water temperature, which must typically be supplied at a temperature in excess of the desired output temperature for the physiological fluid. Further, additional tubing and a drip chamber is required between the heat exchanger device and the patient, which will result in the blood losing heat as it passes to the patient and further make the delivery temperature of the blood difficult to predict and control.
Finally, U.S. Pat. No. 3,881,483 issued to Sausse discloses a device for heating blood as it leaves a blood oxygenator. The heating device consists of an electrical heating element which is in direct contact with the blood carrying tube, which is an undesirable arrangement since it could potentially expose the patient to an electrical shock.
In general, physiological fluid heaters are limited by the temperature to which the fluid may be heated. For example, blood should not be heated to temperatures greater than 41.degree. C. in order to avoid destruction of important components of the blood, such as red blood cells. Thus, there is a limit on the extent to which the temperature of the fluid in the warming device can be raised in order to compensate for any heat lost from the fluid after leaving the warmer and prior to reaching the patient.
There is a need for a physiological fluid warmer which provides high fluid flow rates of up to 160 ml/min while efficiently heating the fluid to a temperature of approximately 37.degree. C. The warmer should also heat the fluid a minimal amount in order to avoid destruction of important fluid components, and provide a minimal distance from the warmer to the patient in order to minimize the heat lost from the fluid after it has been heated. In addition, the warmer should be configured to avoid exposing the patient to any potentially dangerous electrical heating elements.